In recent years, in order to improve the fuel economy performance of an internal combustion engine, it is intended to make lower the viscosity of lubricating oil to be used. However, when the original viscosity of lubricating oil is low, it becomes easy to cause cutting or shortage of an oil film due to a deterioration of the lubricating oil. When an oil film shortage occurs, friction in sliding parts of the internal combustion engine rather increases, thus giving rise to a fear that a large deterioration of fuel mileage may be caused.
In a Patent Document 1, there is disclosed a technique that cools lubricating oil by cooling an oil pan by means of cooling water, thereby preventing the deterioration of the lubricating oil. However, when the lubricating oil is cooled excessively, the viscosity of the lubricating oil goes up to an excessive extent, as a result of which an increase in friction will be caused. In addition, when the viscosity of the lubricating oil goes up to an excessive extent, the work load of an oil pump will increase, which will also become a cause for fuel mileage deterioration.    Patent Document 1: Japanese patent application laid-open No. 2006-168701